Movably clamped doctor blade device and process for evening the coating of

ABSTRACT

An apparatus for blade coating of a travelling web, preferably a paper web, comprising means for supporting and feeding of the web and a doctor blade extending across the web and engaging the web along a first long-side thereof for evening out a coating composition applied to the web and a blade holder carrying the blade along a second long-side, characterized by pressure means arranged to bring, by applying pressure from an outer side of the blade and adjacent to the first long-side, the first long-side of the blade to engagement on the web without imparting substantial bending to the blade as seen in its cross section, said blade being movably clamped in the blade holder; a process for evening the coating of such a traveling web using such an apparatus.

This patent application claims priority from International ApplicationNumber PCT/SE92/00612 with an international filing date of Sep. 4, 1992.

BACKGROUND OF THE INVENTION

The present invention relates to an apparatus for blade coating of atravelling web, preferably a paper web, and the invention also covers aprocess for providing such coating.

In devices for blade coating of travelling webs, in particular paperwebs, the application of the coating composition is controlled by theengagement pressure of the doctor blade against the web, the pressureper unit of surface at the contact area of the blade against the webbeing substantially determining of the applied quantity of coatingcomposition. Since the pressure per unit of surface is decisive for theapplied quantity it is thus essential that the engagement surface of theblade is parallel in relation to the web.

In known processes for such coating the blade is clamped in a bladeholder, the blade being bent or curved as seen in a cross section in aplane extending perpendicular to the web and in the direction of itstravel. By this bending of the blade there is obtained a spring forceacting against the paper web. The most common and easiest way ofaffecting the blade pressure, i.e. to change the spring force, is tovary the distance between the blade attachment site and the travellingweb. In this process the bending of the blade is affected, which alsocauses a change of the spring force of the blade against the web. Thedisadvantage of this process is that the parallelism between the edge ofthe blade and the web as indicated above will be lost, the appliedquantity of coating composition being affected in a non-desired way upto the moment when the edge of the blade has been abraded to renewedparallelism with the web.

In order to avoid this inconvenience in the conventional art methodshave been developed for the purpose of controlling pressure withoutchanging the angle of engagement of the blade. This has been done bybending the blade in such a manner that the spring force will be changedwithout significant change of the angle of engagement of the bladeagainst the web. Such a process is described in Swedish patent8502927-0. The development of paper machines has resulted in processingof web width of up to 8 to 10 m. This development has resulted in greatdifficulties in connection with the use of presently availabletechniques, in view of the fact that with such width of the web it isdifficult to manufacture a blade holder which will not be deformed, i.e.bent, under the influence of varying forces originating from bladepressure, heat tensions, deflection by inherent weight, etc. Since thedoctor blade in the known technique is rigidly clamped in the bladeholder the bending or deformation of the latter will affect both theangle of engagement of the blade and the pressure in its engagementagainst the web in a non-desired manner.

For the purpose of avoiding these problems methods have been developedfor the purpose of enabling continuous measurement of the rectilinearityof the blade holder and automatic compensating for deviations fromrectilinearity while using hydrolic systems. The disadvantage of suchtechnical solution is the fact that it is technically much too advancedand results in high investment costs and, moreover, it requiresextensive maintenance. Techniques are also developed whereby it ispossible to point-wise, automatically or manually, exert pressure on theblade in order to compensate for the variations arising at the area of athe blade engagement when the blade holder is deformed. The disadvantageassociated with these techniques consists in the fact that by exerting avarying pressure on the blade the parallelism between the surface ofengagement of the blade and the web will be affected which has for aneffect that these techniques are technically very difficult to apply.

OBJECT OF THE INVENTION

The present invention has for its object to provide an apparatus, bymeans of which a simple and operationally safe continued control of Theapplied quantity during the course of the coating will be obtained in asatisfactory manner.

Another object of the present invention is to provide a process by meansof which the disadvantages of the prior art are eliminated or at anyrate substantially reduced.

SUMMARY OF THE INVENTION

The present invention is based on a totally new concept for controllingthe engagement pressure of the blade against the travelling web, and theinvention enables control of said pressure of engagement of the bladewithout changing the parallelism of the edge of the blade toward theweb.

For these and other objects that will be clear from the followingdescription the invention provides for an apparatus for blade coating ofa travelling web, preferably a paper web, said apparatus comprisingconventional means for supporting and feeding of the web and a doctorblade extending across the web and engaging the web along one edge orlong-side thereof. The blade has for a function to even out a coatingcomposition applied to the web. The apparatus furthermore comprises ablade holder carrying the blade along its other or opposite long-side.The apparatus according to the invention is characterized by pressuremeans which are arranged to bring said first long-side of the blade,from the above and adjacent To said long-side of the blade, toengagement on the web without imparting substantial bending to the bladeas seen in a cross-section. The apparatus is furthermore characterizedby the fact that the blade is movably clamped in the blade holder.

Said pressure means of the apparatus preferably comprise a ledge or listcoextensive with the blade which in its cross-direction forms an angleto the blade and which is arranged to exert a pressure along said firstlong-side of the blade with one of its longitudinally extending edges.It is, moreover, preferred that said pressure means are arrangeddistributed along the length of the list for individual section-wisecontrol of the pressure against the blade along its said firstlong-side. Through this preferred embodiment of the device according tothe invention the control is suitably carried out automatically byfeed-back from sensors distributed across the width of the web, saidsensors continuously measuring the applied quantity of coatingcomposition. By such continuous control of the quantity of appliedcoating composition across the web possible deformation of the bladeholder will be compensated, said deformation being due to heat stresses,deflection in view of inherent weight, deflection in view of forces fromthe blade pressure, etc.

For the purpose of facilitating this section-wise control of thepressure against the blade it is suitable that the list or ledge isprovided with laterally extending slits opening at the otherlongitudinally extending edge of the ledge, said ledge being impartedthe necessary flexibility.

The blade holder allowing movable clamping of the blade in accordancewith the basic concept of the invention can be designed in two partswith one fixedly anchored lip and a juxtaposed movable lip, the bladebeing movably clamped between said lips.

In order to prevent the coating composition from penetrating into theinterior of the blade holder it is provided that one of said lips isprovided with a resilient sealing arranged along the blade holder, saidsealing simultaneously allowing movability of the blade.

The apparatus according to the invention is suitably provided with spacemeans located between the two lips, whereby the clamping pressurebetween the lips can be controlled.

In connection with the site of evening of the coating composition bymeans of the doctor blade it is suitable to arrange a backing, forexample a roll, the surface facing the web of which is elasticallyresilient. Such resiliency can suitably be provided by an elastic andresilient coating, for example of rubber or similar synthetic material.In this way unecessary web failures can be avoided in view of the factthat bulges, particles in the paper web or the like easily can passbetween the web and the doctor blade.

The invention also provides for a process for coating a travelling webwhile using a doctor blade engaging the web, said doctor blade extendingacross the web, for example across its entire breadth, and said bladehaving for its function to even out a coating composition applied to theweb through engagement against the web of one longside or edge of theblade. Said process is characterized by the fact that the engagement ofthe blade is provided by application of forces from the outside alongsaid longside of the blade so that the blade can exert its eveningfunction without substantial bending in its cross-direction.

It is preferred in this process that said forces along the long-side ofthe blade are controlled section-wise independently of each other forproviding even coating across the whole breadth of the web.

The expression used in the present disclosure "adjacent to" with regardto the site of applying the forces from the outside of the doctor blademeans that said forces are applied sufficiently close to the blade edgein question in order that the blade shall remain substantially straightand not curved as seen in a cross-section parallel to the direction ofthe travel of the web.

The invention will in the following be further described in connectionwith a preferred embodiment, and said description is made with referenceto the appended drawings, wherein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows diagrammatically the traditional techniques of obtainingblade engagement against web and backing;

FIG. 2 illustrates diagrammatically the technique according to thepresent invention for obtaining a corresponding engagement against theweb;

FIG. 3 shows the basic design of the apparatus according to theinvention more in detail;

FIG. 4 shows a section A--A of the apparatus of FIG. 3; and

FIG. 5 illustrates a detail on the area around the site of the bladeengagement when using a resilient backing.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the traditional way of obtaining engagement of the doctorblade against a travelling paper web. In this prior art the blade isdeflected by its fixed clamping in a blade holder, and the springproperties of the blade provides the engagement pressure F₁ against theweb and the backing which is desirable for providing the intendedevening out of the coating composition that has been applied to the web.The desired engagement pressure against the paper web will be obtainedby adjusting the distance a) between the blade holder and the backing 1through supplementation with an outer load F₂ applied to the doctorblade, whereby the total engagement pressure is the sum of the pressureobtained by the fixed clamping of the blade and the application of theexternal load.

FIG. 2 illustrates diagrammatically the technique according to thepresent invention, where the engagement pressure against the paper webis provided without making use of spring properties of the doctor blade.In this technique the blade holder generally designated 5 is positionedat a constant distance a) to the backing 1. Blade 3 remains essentiallystraight in view of the movable clamping in the blade holder 5. Byforces F₂ applied at the free long-side of the blade 3 the desiredengagement against the web and the backing will be provided withoutbending blade 3 and without losing the parallelism vis-a-vis the paperweb. Forces F₂ are applied for this purpose adjacent to the site ofengagement between the free edge of doctor blade 3 and the web and thebacking.

FIGS. 3 and 4 show more in detail an exemplifying embodiment of theapparatus according to the invention. At the top and to the left in FIG.3 there is shown a minor part of the backing in the form of a resilientcoating 2 on a roll not shown, said coating being for exampleconstituted by rubber of the suitable hardness. The blade holder propergenerally designated 5 includes a fixed lip 21 which is attached to thefixed parts of the coating equipment in a manner not shown, and ajuxtaposed movable lip 23. Said movable lip 23 is pivotable around a pin27 arranged in two side elements 25 placed on each side of the movablelip 23. Both lips 21 and 23 thus extend along the whole width of theweb.

The blade holder 5 has for a function to hold a doctor blade 3 with itsrespective long-sides 4, 6 between the two lips 21, 23 in a movablemanner. Also this blade extends across the whole width of the paper weband engages by longside 4 the paper web and the underlying backing 9. Inthe embodiment shown the upper lip 21 is provided on the interior sidethereof with a round bar 29 placed in a corresponding recess, said roundbar 29 extending across the whole length of the lip 21. On the oppositeside and on the inside of the movable lip 23 a resilient rubber list 31is provided placed in a corresponding recess, and also said rubber list31 extends along the whole length of blade holder 5 or lip 23. By thisarrangement both sealing against the coating composition and desiredmovability for doctor blade 3 will be obtained. By the use of stop knobs33 attached to the edge of blade 3 and positioning pins 35 easy assemblyand determination of a location of doctor blade 3 will be enabled. Byadjustable spacers 37 the nip pressure between the engagement surfacesof the lips can be controlled.

The necessary external compressive force on the doctor blade 3 adjacentto the engagement of the blade against the web will be provided by alongitudinally extending pressure ledge 41 which is attached to a numberof pivot arms 43 distributed across the width of the web. These pivotarms 43 are positioned at such intervals that pressure ledge 41 will notbe subjected to substantial deflection in the space between each pivotarm 43. Pressure ledge 41 is provided with slits 49 distributed alongits length for the purpose of imparting to the pressure ledge suitableflexibility in connection to varying pressures as determined inassociation with each pivot arm 43.

Each pivot arm 43 is pivotably arranged on lip 23 around pins 45attached to side elements 47 placed on each side of each pivot arm 43and in turn attached to the outer side of lip 23. In connection to eachpivot arm 43 there is arranged a pressure member 51 in the form of abellows acting on the free end of pivot arm 43 via plates 53, 55attached to the outer surface of lip 23 and to the inner free end ofpivot arm 43, respectively. In view of the fact that each pressuremember 51 can be controlled individually it is thus possible to controlthe engagement pressure of blade 3 against the web section-wise alongthe length of blade 3. In this manner one can ensure uniformity ofcoating of coating composition on the paper web across the full widththereof and in its longitudinal direction.

FIG. 5 shows diagrammatically in an enlarged view the area around theengagement of blade edge 4 on the paper web 7. Thus, when there areirregularities in the coating composition 11 or in the paper web 7 suchirregularities can easily pass the area between the engaging edge of theblade and paper web 7 in view of the fact that backing 2, in view of itsflexibility, yields to the formation of a depression. The risk for webfailure with concomitant operational disturbances will hereby bereduced.

The invention is, of course, not limited to the embodiments shown, sincealternative examples of embodiments are conceivable. Thus, theattachment of the doctor blade can be carried out in many ways meetingwith the requirement of movability of the doctor blade. Also thetechnique of exerting an external load on the blade thereby controllingits engagement against the web can be modified. It is only essentialthat the blade is not subjected to substantial bending or curving underthe influence of forces applied exteriorly, whereby the parallelismbetween the engaging edge of the blade and the web can be maintained. Inaddition the invention enables section-wise controlling of theengagement pressure across the web and, of course, also possibility ofcontrolled section-wise pressurizing so as to obtain an even coatingalso in the direction of web travel.

I claim:
 1. A process for evening out a coating composition on atravelling web while using a doctor blade engaging the web, comprisingthe steps of:providing a doctor blade including a first long-side, asecond long-side, an inner side and an outer side, said doctor bladeextending at least partially across the web, and a blade holder carryingthe blade (3) along the second long-side, said doctor blade beingmovably clamped in the blade holder so as to allow the blade to moveindependent of blade holder movement and evening out a coatingcomposition on a travelling web through engagement against the web ofthe first long-side of the blade, characterized in that the engagementof the blade is provided by an application of forces on the outer sidealong said first long-side of the blade so that the blade can exert anevening function without sufficient bending in its cross-direction thatresults in the first long-side of the blade losing parallelism with theweb.
 2. A process according to claim 1, characterized in that saidforces along the long-side of the blade are controlled section-wiseindependently of each other for providing even coating across the web.3. The process of claim 1, wherein the blade extends across an entirebreadth of the web.
 4. The process of claim 1, wherein the movement ofthe blade independent of blade holder movement is a rocking motiontowards and away from the web.
 5. An apparatus for blade coating of atravelling web (7), comprising:means (1, 2) for supporting and feedingof the web; a doctor blade, said doctor blade comprising a firstlong-side, a second long-side, an inner side, and an outer side,extending across the web and engaging the web along the first long-side(4) of the blade for evening out a coating composition (11) applied tothe web; a blade holder (5) carrying the blade (3) along the secondlong-side (6), said blade being movably clamped in the blade holder soas to allow the blade to move independent of blade holder movement; andpressure means (41, 43, 51) for applying pressure on the outer side andadjacent to said first long-side so as to bring said first long-side ofthe blade to engagement of the web (7) without imparting substantialbending to the blade thereby maintaining parallelism between said firstlong-side of the blade and the web.
 6. An apparatus according to claim5, characterized in that said pressure means comprise a ledge (41)coextensive with the blade which in its cross-direction forms an angleto the blade (3) and which is arranged to exert with a firstlongitudinally extending edge thereof, a pressure against the bladealong said first long-side of the blade (3).
 7. An apparatus accordingto claim 6, characterized in that said pressure means (43, 51) arearranged along a length of the ledge (41) for individual section-wisecontrol of the pressure against the blade (3) along said first long-sideof the blade (4).
 8. An apparatus according to claim 7, characterized inthat said ledge (41) is provided with laterally extending slots (49)opening at a second longitudinally extending edge (6) of the ledge forthe purpose of imparting flexibility to the ledge in connection withsaid individual pressure control.
 9. An apparatus according claim 5,characterized in that the blade holder includes a fixedly anchored lip(21) and a juxtaposed movable lip (23), the blade (3) being movablyclamped between said lips (21,23) so as to allow the blade to moveindependent of blade holder movement.
 10. An apparatus according toclaim 9, characterized in that at least one of said lips is providedwith a resilient sealing lip, whereby there is obtained both sealingagainst the coating composition and the movability of the blade (3)independent of blade holder movement.
 11. An apparatus according toclaim 10, characterized in that space members (37) are arranged betweenthe two lips (21, 23), whereby a clamping pressure between the lips canbe controlled.
 12. An apparatus according to claim 9, characterized inthat at least one of said lips is provided, on an interior side thereof,a round bar (29) placed in a corresponding recess, thereby obtaining themovability of the blade independent of blade holder movement.
 13. Anapparatus according to claim 5, characterized in that said means forsupporting and feeding of the web (7) comprise a backing (1, 2) arrangedin connection with an area of evening out the coating composition (11)by means of the doctor blade (3), the surface of said backing facing theweb being elastically resilient.
 14. The apparatus of claim 13 whereinthe backing is a roll.
 15. An apparatus according to claim 13,characterized in that said backing (1) is provided with a resilientcoating (2).
 16. The apparatus of claim 15 wherein the resilient coatingis comprised of rubber.
 17. The apparatus of claim 5 wherein thetravelling web is a paper web.
 18. The apparatus of claim 5, claim 9,claim 10 or claim 12, wherein the movement of the blade independent ofblade holder movement is a rocking motion towards and away from the web.